1. Field of the Invention
The present invention relates to a calcining tool material for use in the calcining of powder metallurgical products or functional ceramics.
2. Description of the Related Art
It has heretofore been a common practice that a calcining tool material for use in the calcining of powder metallurgical products or functional ceramics is coated with a dense film having a low reactivity in order to inhibit the reaction of components in the calcining tool material with the material to be calcined or the absorption of components contained in the material to be calcined by the calcining tool material that would causes the deterioration of the material to be calcined. As the material to be used in such a film there has been widely used zirconia having a low reactivity. As the substrate for calcining tool material there has been widely used an alumina-silica-based material taking into account the cost and durability.
Examples of method for forming a coating layer as mentioned above on the surface of such a calcining tool material include a method which comprises forming a substrate, optionally calcining the substrate, casting or spraying a slurried coating material onto the surface of the substrate, and then calcining the coated material, a method which comprises fixing a coating material to the same substrate as mentioned above with an adhesive, and a method which comprises forming a coating layer on a substrate by a flame spraying method while being baked.
Particularly desirable among these methods is the flame spraying method because it can form a dense coating layer. When used repeatedly, even the calcining tool material having a coating layer formed thereon by the flame spraying method can be subject to peeling of the coating layer that causes the substrate to be exposed, disabling desired calcining.
The peeling of the dense coating layer formed on the surface of a calcining tool material by flame-spraying method is mainly attributed to the difference in thermal expansion coefficient between the substrate and the flame spray coating layer or the residual expansion of the coating layer caused by the destabilization of the coating layer due to the incorporation of stabilized or partially-stabilized zirconia. Various countermeasures have heretofore been worked out against the peeling of the coating layer attributed to the difference in thermal expansion coefficient the substrate of the calcining tool material and the coating layer formed thereon. However, no significant countermeasures have been worked out against the peeling of the coating layer attributed to the residual expansion of zirconia caused by the destabilization of zirconia.
JP-B-3-77652 (The term "JP-B" as used herein means an "examined Japanese patent publication") proposes that stabilized zirconia having CaO content of from 4 to 31% by weight be flame-sprayed onto the surface of an alumina-silica-based substrate with Al.sub.2 O.sub.3 content in the substrate and the content of a stabilizer in zirconia controlled such that the thermal expansion coefficient of zirconia almost coincides with that of the alumina-silica-based substrate. However, an alumina-silica-based substrate has CaO content of about from 4 to 5% by weight at largest from the standpoint of thermal expansion coefficient and reactivity with the material to be calcined. Thus, the content of CaO in the stabilized zirconia can be hardly raised any more. Further, if a calcining tool material coated with a flame spray coating layer of stabilized zirconia having such a single composition is used many times, the coating layer is often peeled mainly due to the residual expansion of zirconia part.
JP-B-4-586, which is another prior art, proposes that stabilized zirconia having CaO content of from 4 to 15% by weight be flame-sprayed onto an alumina-based substrate in such a manner that CaO content decreases from the surface of the alumina-based substrate toward the surface of the coating layer. In some detail, the coating layer has higher CaO content toward the substrate to have a thermal expansion coefficient as high as that of the substrate. On the contrary, the coating layer has lower CaO content toward its surface to have a lower thermal expansion coefficient. According to this proposal, flame spraying is effected batchwise in a plurality of layers, adding to cost. Further, these layers each form a flame spray coating layer of a single stabilized zirconia. Therefore, such a calcining tool material is disadvantageous in that when used many times, the coating layer is peeled mainly due to the residual expansion of zirconia part.
JP-B-4-21330 proposes that a flame spray coating layer of alumina be provided interposed between an alumina-based substrate and a flame spray coating layer of zirconia to reduce the difference in thermal expansion coefficient between the substrate and the zirconia layer. In some detail, the thermal expansion coefficient at 1,000.degree. C. decreases in the order of stabilized zirconia, partially-stabilized zirconia, alumina and alumina-based substrate. Thus, an attempt was made to reduce the difference in thermal expansion coefficient between the substrate and the alumina layer by providing an alumina layer interposed between the substrate and the flame spray coating layer of zirconia. However, thisproposal, too, requires two or more kinds of flame spraying steps, adding to cost. This proposal is also disadvantageous in that the coating layer, if it is made of stabilized zirconia or partially-stabilized zirconia, is peeled due to the residual expansion similarly to JP-B-4-586.
JP-A-5-178673 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes that the same flame spray coating layer of alumina as in JP-B-4-21330 be provided interposed between an alumina-silica-based substrate and a flame spray coating layer of unstabilized zirconia. However, this proposal, too, adds to cost. This proposal is also disadvantageous in that the coating layer, if it is made of unstabilized zirconia alone, is liable to remarkable weakening of structure attributed to abnormal expansion and shrinkage due to transformation characteristic of unstabilized zirconia caused by repeated heat history during use. Thus, the calcining tool material thus proposed is soon worn to tatters.